Medulloblastoma is the most frequent solid tumor malignancy in the central nervous system of children. Treatment of medulloblastoma is associated with neurological and endocrine side effects that result in a poor quality of life for survivors. Advances in the treatment of this disease have been limited by the absence of an appropriate mouse model. The applicant bases this project on preliminary studies that characterized a mouse model for medulloblastoma. Heterozygous loss of Patched-1 (Ptc1), a membrane receptor for Sonic hedgehog (Shh), an extracellular protein that inhibits signaling by Smoothened (Smo), another membrane protein that increases expression of the zinc finger transcription factor Gli1, causes the developmental disorder Gorlin syndrome, which is associated with an increased incidence of medulloblastoma. Approximately 95% of mice in which one copy of Ptc1 and both copies of p53 are deleted develop medulloblastoma by 12 weeks of age. Three specific aims are proposed to address the role of the Ptc1 pathway in tumorigenesis. In aim 1, a series of pharmacological inhibitors will be used to investigate the contribution of the pathway to proliferation, differentiation, response to DNA damage, patterns of gene expression and tumorigenesis in medulloblastoma cells. The results of these studies will be used to test intervention strategies based on the use of pharmacological inhibitors to block tumor growth in mice. In aim 2, a structure/function analysis of Gli1, a transcriptional target of the Ptc1 pathway will be undertaken. This involves the use of gain of function and loss of function mutations in Gli1 to identify target genes in tumor cells. In aim 3, the contributions of epigenetic and genetic alterations in medulloblastoma will be investigated by transplanting medulloblastoma nuclei into oocytes. This approach will reveal whether medulloblastoma nuclei can be reprogrammed to contribute to normal development in blastocysts, embryos and mice.